Rear projection systems are currently one of the most popular types of large-screen display systems that are widely available to the general public. In order to generate images, these projection systems will typically employ a spatial light modulator such as a light valve. A light valve will typically employ a dense array of valvelets, each of the valvelets may contribute to the formation of image portions.
As described, a valvelet is typically used to generate a portion (“image segment”) of an image, the portion generated may include a pixel, or a plurality of pixels of the image to be produced. In order to generate image segments, each valvelet may be manipulated into at least two orientations, an “on” orientation, in which light is either passed or reflected to the display screen, or in the “off” orientation, in which case no light is allowed to pass or reflected to the display screen such as pulse width modulation control scheme (usually associated with MEMS devices like the DMD). By controlling the number of times a valvelet is oriented in the “on” position per unit of time, the brightness of the image segment being generated may be controlled. The more often a valvelet is in the “on” orientation per unit of time, the brighter the corresponding image segment will be. In analog control scheme (usually associated with liquid crystal device (LCD) and liquid crystal on silicon device (LCOS)) the device can be partially turned on and thus transmit only a portion of the light. The valvelet can control the percentage of light transmitted or reflected to the screen. When combined, the resulting image segments produced by these valvelets will form an image frame that is eventually projected onto a display screen. The image frames formed, in some instances, may be single color image frames that may be associated with the primary colors of red, green and blue. When these single color image frames are projected onto and combined on the display screen, they may form one or more full color images.
The variations in brightness of an image segment projected onto a display screen, in this context, can vary from full color when valvelets are in the “on” orientation the maximum number of times or the maximum transmission possible per unit of time, to black, when the valvelets are left in the “off” orientation. For simplicity, variation between full color and black for any one color is defined as different shades of “gray”. For example, in an 8-bit video image, there may be 256 shades of gray from full color to black. These different shades of gray may correspond to the number of times (i.e., counts) a valvelet can be in the “on” position per unit time period or the percentage of maximum transmission or reflection in the case of an analog drive. In an 8-bit video, shades of gray may range from full color when the count is at a maximum of 255 to black when the count is 0. Note that other video formats will have different ranges of gray. For example, in contrast to the 256 shades associated with 8-bit video image, a 10-bit video image can have 1024 shades of gray. For purposes of this description, the number of times (i.e., counts) that a valvelet is in the “on” orientation per unit of time or the percentage of full transmission/reflection will be referred to as the “grayscale value.” Thus, for the shade representing the maximum white or full color, the grayscale value would be 255.
FIG. 1A depicts a projection system employing a particular type of Fresnel lens, which is disposed on the back of a display screen 15. The type of Fresnel lens 16 that is depicted here extends across the entire length of the display screen 15 and contains many grooves that act to redirect light that is projected onto the lens 16 at sharp angles to the viewer. The system 10 includes an image source 12 that generates one or more images that are directed to a series of mirrors 14. The one or more images are then eventually projected onto the Fresnel lens 16 (i.e., display screen 15), which redirects the one or more images to the viewer via the display screen 15. The image source 12 typically includes a number of components such as a light valve, one or more illumination sources such as an arc lamp and/or light-emitting diodes (LEDs), and assorted lens and mirrors.
FIG. 1B is a plan view of the Fresnel lens 16 of FIG. 1A. In this illustration, the display screen 15 is located on the backside of the Fresnel lens 16. As can be seen, the grooves form concentric semicircles. The letters A1, A2, A3, B, C, D, and E indicates various locations or positions on the Fresnel lens 16 (as well as on the display screen 15). These lens or screen locations will also be associated with specific valvelets of a light valve or valves that may be used to produce the image segments to be projected onto these locations.
FIG. 1C depicts an example array of-valvelets on a light valve that may be used to project image segments onto the Fresnel lens 16 of FIG. 1B. Note that the valvelet array 30 is not depicted to scale as it relates to the Fresnel lens 16 and will be typically much smaller than the Fresnel lens 16 depicted in FIG. 1B. Note further that a typical valvelet array will include many more valvelets than the amount depicted in FIG. 1C. Valvelet VA1, VA2 and VA3 corresponds to the valvelets that are used to generate the image segments that are projected onto the Fresnel lens (or screen) locations designated as A1, A2, and A3 in FIG. 1B. Similarly, VB, VC, VD, and VE correspond to the valvelets that are used to generate the image segments that are projected onto the locations designated as B, C, D, and E in FIG. 1B.
One problem associated with current projection systems, such as the one depicted above, is the brightness nonuniformity of images that are projected onto the display screens. This may be as a result of several factors including the presence of certain components along the optical paths of the projection systems. For example, the use of a projection lens or other similar devices may result in lower than desired brightness for certain segments of the image to be projected onto the display screen due to vignetting in the lens and lower transmission for light reflected/transmitted from the valvelets farthest from the optical axis of the projection lens. For example, in rear projection systems that employ a diffusion screen without a Fresnel lens, certain portions of the image to be generated will be darker or dimmer than ideal due to, for example, higher surface reflections at various locations of the display screen. In these projection systems, some light is lost from some of the image segments before the image segments are actually transmitted through the display screen and to the viewer. Typically these darker or dimmer regions may be associated with, for example, the corner regions of the image or images to be generated on the display screen (see ref. 20). Consequently, these dimmer regions can also be associated with specific locations on the display screen (e.g., corner locations of the screen). In one case where Fresnel lenses of a particular design is employed, a dim region will also be present at the bottom center of the display screen (see refs. 22). Thus, the dimness of a specific image region will be dependent upon several factors including the type of components being used (e.g., type of Fresnel lens employed) and its location relative to the display screen 15.
In FIG. 1B, the locations A1, A2, A3, B, C, D and E are screen locations that are associated with varying darkness or dimness for a particular type of Fresnel lens 16. In this example, the letters A1, A2, and A3 corresponds to the locations (on the Fresnel lens 16 as well as on the display screen 15) where the darkest image regions will be located. Letters C, D, and E correspond to the screen positions that are associated with image regions that are, in terms of brightness, between the darkest region (A1) and the brightest region (B). As a result of this brightness nonuniformity, the image or images that are projected onto the display screen will be less than ideal. For example, suppose in the case of the above system, two image segments of an image having the same brightness are to be projected onto two different screen locations. However, as a result of brightness non-uniformity between the two screen locations, the actual image segments produced at those two screen locations will have unequal brightness. This may result in the overall brightness nonuniformity of the image to be generated.
At this time, it should be noted that there are other types of Fresnel lens other than the Fresnel lens 16 depicted above. These other Fresnel lens types will typically have different topography of contrasting dimness and brightness then the dimness and brightness depicted above for Fresnel lens 16. For example, in one type of Fresnel lens, the region A1 in Fresnel lens 16 (which was one of darkest regions on the lens) may not be the darkest region for the same corresponding region of another type of Fresnel lens. In fact, in other types of Fresnel lens, the exact distribution of darkest and lightest regions may be completely different from those of the Fresnel lens 16 depicted above in FIG. 1B. In such Fresnel lens, the brightest location (such as location B on Fresnel lens 16 above) may be located in other locations such as at locations corresponding to locations C or D of Fresnel lens 16 depicted above. In yet other types of Fresnel lens, there may not be a central dark region as depicted in FIG. 1B (see A1).